1. Field of the Invention
This invention relates to foam materials produced from a chemical reaction by mixing two liquid components, and in particular to a system for dispensing the foam material and applying the foam material to a substrate.
2. Description of the Prior Art
Two component foaming systems are used in a wide variety of applications to create a closed-cell foam product and to apply the foam product to a substrate. Foam products produced in this manner can be used as sealants which provide barriers against air, dust, vapor and fluids in various applications. They also find application in providing thermal or sound insulation layers.
In two component foam dispensing systems, the foam is created by a chemical reaction between the two liquid components. When the two liquid components are mixed together, the chemical reaction occurs, creating a new compound, such as polyurethane, which is the foamed material, and creating a foaming agent, such as carbon dioxide, which is entrained within with the foamed material to create the foam. Since the chemical reaction occurs almost instantaneously, the two liquid components must be mixed together in a dispensing head immediately prior to applying the foam material onto the substrate.
In order to properly and effectively create and apply two component foam products, several factors must be considered. One consideration is the proper ratio of liquid components. In order to create the foam product with the desired physical properties and appearance, the mass quantities of each of the components must be properly matched. This requires that the flow rate of each of the liquid components that is supplied to the mixing and dispensing head be accurately controlled. Since the mass flow of each component will vary depending upon several factors, such as the amount of entrained air in the liquid and the temperature of the liquid, it is sometimes difficult to accurately meter the flow of the components to the dispensing head.
Another consideration is the separate maintenance of each of the components prior to dispensing and the proper mixing of the components in the dispensing head. Since the components when mixed will almost instantly react to produce the foamed product, it is important that the components remain separate and that mixing of the components is avoided until immediately prior to dispensing the foam onto a substrate. For this purpose, a dispensing head or gun must be used in which the two components can be mixed and in which the mixture is then immediately applied to the substrate.
The mixing and dispensing heads used with such systems can be either dynamic mixers or static mixers. Dynamic mixers using a mixing element, such as a rotating mixing core to create a turbulence in a mixing chamber which contains both components. The components are thus mixed together, and the chemical reaction which creates the foam product occurs. Static mixers accomplish the mixing of the components without a moving element. An example of static mixer is an impingement mixer in which the mixing of the two liquid components is accomplished by forcing each component through a jet or nozzle and shooting each component against each other to mix the components together.
Another consideration is the maintenance of the dispensing head or gun in a clean condition. After the liquid components have been mixed together in the head, the foam product is created, and this foam product can build up along the dispensing flow path inside the gun. This build-up of foam material must be cleaned from the gun periodically, and for this purpose a purge system is usually provided. The purge system is used to provide a supply of an appropriate cleaning solvent to the dispensing flow path of the gun to clean the flow path.
Another consideration is the ability of the system to accommodate intermittent operation of the dispensing head or gun. In most dispensing systems, the foam product is not dispensed continually onto the substrate. Instead, the dispensing flow is turned on and off, and this intermittent operation of the dispensing head must be accommodated. When the dispensing flow is turned on and off, there can be intermittent increases in pressure of the lines supplying the liquid components to the dispensing head. These pressure increases can affect the flow rate of the components, and, if the mass flow rate of one component varies relative to the other component, the components will not be mixed in the proper proportion, causing an adverse effect to the properties and appearance of the foam product being dispensed.
One problem associated with the mixing and dispensing heads is the residual dripping of the foam product from the dispensing head after the flow of foam material should be cut off The flow of foam material in the mixing and dispensing head is usually regulated by valves which control the supply of the liquid components to the head. When it is desired to interrupt the dispensing of the foamed material, the component supply valves are closed. The remaining amounts of liquid components mix together to create the foamed product, which then exits from the mixing chamber and from the dispensing head, but there is usually no positive shut off of the foamed material from the dispensing head. As a result, small remaining amounts of the foam product can continue to drip from the dispensing head after the flow of the components has been cut off. This drip can fall onto portions of the substrate at undesired locations and can otherwise provide a nuisance during production.